An optical sensor that uses an optical fiber to guide light to a measuring unit, and senses changes in the optical condition at the measuring unit, does not use electricity in the measuring unit; it therefore has advantages such as offering excellent resistance against explosion, lightening, and electromagnetic noise, and facilitating long-distance measuring. In this type of optical sensor, when the target being measured is a physical quantity other than temperature, changes in characteristics caused by changes in temperature can reduce the precision of the measuring. Therefore, to achieve high-precision measuring, effects of temperature-change must be compensated for.
Conventionally, techniques such as temperature-compensated optical fiber sensor methods using optical interference (See Patent Documents 1 and 3), and methods that measure changes in the center wavelength of a fiber grating (See Patent Document 4), are proposed. However, since a principle of these conventional techniques is to measure wavelength changes and the modulation component of light, these measuring devices are expensive Other factors making them expensive are that they require a variable-frequency light source to realize temperature-compensation (Patent Document 1), that they require a special structure (Patent Document 4), and so on.
Also proposed is a method that allows use of a less expensive measuring device by measuring changes in light power (e.g. see Patent Documents 2, 5, and 6). This method makes a target being measured reflect light emitted from an optical fiber that guides light from a light source to a sensing unit, and measures the power of light coupled in a same optical fiber.
This method has advantages that the configuration of the sensing unit is simple, and the measuring unit for measuring changes in light power can be provided comparatively inexpensively.
However, in this method, since light is also received in an optical fiber for projection, a light-branching element such as an optical coupler must be provided to make the reflected light incident to a light-receiving unit such as a photodiode. Since this light-branching element has temperature-dependency and light-source wavelength-dependency, its branching ratio changes according to changes in temperature and the light-source wavelength. As a result, the configurations of Patent Documents 2, 5, and 6 are problematic in that change in the light-branching ratio affects the measurement value, and thus reduces the measuring precision.
To avoid this problem, Patent Document 5 attempts to stabilize the measuring precision by adding a mechanism that ensures a constant temperature at the light-branching unit, but this is problematic from a practical point of view, since it complicates the configuration and increases the cost by requiring a temperature-controlling mechanism.
A method where light emitted from the optical fiber is received in another fiber (e.g. see Patent Documents 7 to 9) is also proposed. This method can measure light power without using a light-branching element.
However, in Patent Documents 7 and 8, to receive a large light power, a light-receiving fiber is made by bundling an optical fiber for guiding light with a multimode fiber and a plurality of optical fibers, thereby making modes in the optical fiber liable to change according to temperature, changes in the wavelength of the light source, and external pressure on the optical fiber. Since these mode-changes lead to a decline in measuring precision, highly precise measuring becomes difficult.
In Patent Document 9, a detecting unit uses a lens system to receive a large light power. The configuration consequently becomes complex and more expensive; in addition, there is a problem that measuring precision is liable to decline due to the external environment (temperature, vibrations, etc.).
As explained above, the prior art has not realized a temperature-compensated optical fiber sensor that measures temperature inexpensively and highly precisely, and then uses that temperature data.
Patent Document 1: Japanese Patent Application, First Publication No 60-50402
Patent Document 2: Japanese Patent Application, First Publication No. 5-196528
Patent Document 3: Japanese Patent Application, First Publication No. 9-005028
Patent Document 4: Japanese Patent Application, First Publication No, 2002-267557
Patent Document 5: Japanese Patent Application, First Publication No. 2002-372472
Patent Document 6: Japanese Patent Application, First Publication No. 8-62080
Patent Document 7; U.S. Pat. No. 5,017,772
Patent Document 8: U.S. Pat. No. 4,479,717
Patent Document 9: U.S. Pat. No. 6,433,350